HP Indigo Digital Offset
Color Technology
Table of contents
Introduction
What is HP Indigo Digital Offset Color?......................................................2-3
The HP Indigo range of digital printing presses, based on its Digital Offset
Color technology and process, offers a unique combination of best print
quality in the industry, wide color gamut, substrate versatility, speed,
productivity, flexibility, and the ability to vary every printed copy. HP
Indigo printing inherently matches and at times, due to its digital local
image processing and ink coverage flexibilities, surpasses offset, and is
also the best replacement for photo silver halide applications.
HP Indigo ElectroInk..........................................................................................4
Thermal Offset..................................................................................................5
Color switching..................................................................................................5
The printing cycle..........................................................................................6-7
Quality characteristics .............................................................................. 8-11
Substrate compatibility................................................................................. 12
Productivity and versatility........................................................................... 13
Future capabilities.......................................................................................... 14
Summary......................................................................................................... 15
This white paper describes the HP Indigo Digital Offset Color printing
process and its unique features. It details the latest advances in HP
Indigo’s liquid electrophotography (LEP) technology released for drupa
2016, including dozens of different hardware, supplies and software
enhancements that extend the performance of HP Indigo’s underlying
core technology. It also compares HP Indigo’s liquid electrophotography
with dry toner electrophotography (DEP) and inkjet technology-based
digital printing presses, as well as with the conventional, non-variable,
offset lithography process that has traditionally dominated the printing
industry worldwide.
HP Indigo offers a wide range of digital presses for a variety of
applications, all based on the basic principles of its Digital Offset Color
technology.
2
Figure 1 HP Indigo 12000 Digital Press
1. Digital
The printed image is created directly from digital data, avoiding the use of
any ”analogue” intermediate media. It starts with digitally created pages
or print elements containing, for example, text, layouts or images. Then,
unlike conventional printing processes, there are no intermediate prepress processes between the digital document file and the final print. No
film, no imagesetters, no plates, no platesetters, no photo-chemicals, and
no other ”analogue” elements. There is also no press make-ready: no
plate mounting, no registration adjustments, no ink keys, and no waste.
The HP Indigo process is fully digital from image creation to printing. And,
since it is a fully digital process, every image can be a new one, enabling
information to be varied as required.
2. Offset
Offset simply means that there is an intermediate cylinder that transfers
the ink image from its origin on the plate cylinder to the final substrate
(i.e. the paper, plastic or other material) for printing. In the printing
industry, the term offset is commonly used as a term for the lithographic
process. Indeed, modern lithographic (also called litho) presses do use an
offset process.
HP Indigo technology also uses offset printing, by the use of an offset
cylinder covered with a renewable rubbery blanket.
There are two main purposes of offsetting in printing presses. First, it
protects the surface of the printing plate from excessive wear due to
friction with the substrate as it is printed. Second, since the rubbery
blanket conforms to the local topography of the substrate, ink is adhered
both to the “peaks” and the ”valleys” of the substrate equally. In other
words, it acts as a kind of shock absorber and pressure pad, ensuring
good ink transfer from printing plate to the substrate. Conventional offset
presses can therefore print on a very wide range of substrate surfaces
and thickness, and are superior to non-offset processes in this respect.
The HP Indigo process uses offset for the same reasons, thus making it
capable of printing on a wide range of substrates.
A notable difference between conventional offset and HP Indigo digital
offset printing technology is that HP Indigo’s ink-ElectroInk-transfers from
the blanket to the substrate with virtually none of the ink splitting that
characterizes conventional offset printing systems. This enables the
creation and transfer of a different image each printing cycle.
A further difference is that HP Indigo ElectroInk is dried on the blanket and
the final image is transferred to the substrate in the form of a ready dry
film. This process does not depend on the final substrate. Thus, there is
almost no limitation to substrate flexibility, and all substrates are printed
with the same high HP Indigo quality and at the same speed. Moreover,
since no drying is required the substrate does not undergo any further
stress. In comparison, other technologies require further drying of the ink
on the substrate (as in offset or inkjet technologies) or fusing of toner on
the substrate (as in DEP) and require the matching of ink/toner and
printing process with media.
The matching requirement, at times, affects productivity due to the need,
for example, to wait for drying or to wait for stabilization of the heating
mechanism. When an ink-process-media match does not exist, some
media has to be excluded.
Figure 2
Schematic comparison of color gamut with
4-, 6-, 7-, and 11-color HP Indigo ElectroInks
Conceptual Diagram
Bright Yellow
ProcessYellow
Orange
6 IndiChrome
on-press
7 IndiChrome
Plus
4 Colors
Process
Magenta
Process
Black
Green
*
a
What is Digital Offset Color?
Let’s break it down word by word:
11 IndiChrome
off-press
Rhodamine
Process Cyan
Violet
PANTONE Colors®
4 colors (CMYK)
Reflex Blue
b*
HP IndiChrome on-press (CMYKOV)
HP IndiChrome Plus (CMYKOVG)
HP IndiChrome off-press (CMYKOVG and reflex blue, bright yellow, rhodamine red,
and transparent)
The above differences result in superior versatility of LEP, leading to
higher productivity, due to the ability to change data, color, and substrate
at will and with no time penalty.
3. Color
As it sounds, HP Indigo technology enables digital printing in full color.
However, unlike conventional offset litho color presses, which require one
complete printing unit per color, HP Indigo presses print multiple colors for
each single pass of the substrate through the press. As described above, in
Digital Offset Color technology all the ink transfers from the blanket to the
substrate. The HP Indigo digital press transfers with each rotation of the
press cylinders, on the single set of blanket and imaging plate, not only a
different image but also a different ink. HP calls the method or
configuration “on-the-fly color switching.” This is all done without physical
interaction between the different color separations. Printing with 5, 6 or 7
colors, in addition to CMYK together with the flexibility of adding pre-mixed
HP IndiChrome spot colors, and the capability to vary the number of
impressions per single color, offers major enhancements in color quality,
range, fidelity, and luminosity (see figure 2).
In summary, HP combines digital, offset, and color into a powerful printing
process. The three core technologies of the HP Indigo Digital Offset Color
process are:
•• ElectroInk - HP Indigo’s liquid ink
•• Thermal offset transfer
•• On-the-fly color switching
3
Figure 3
Digital Offset Color core technologies
•• Substrate versatility
•• No Fusing
•• No Drying
Thermal
Offset
Color
switching
•• Color/speed versatility
•• Compact
HP Indigo
ElectroInk
•• High speed
•• Efficient use of material
•• Negligible media interaction
•• Multi-layer (hit) flexibility
•• Extreme saturation possible
Superior image quality
•• Gloss uniformity
•• Sharpness
•• Transparency
•• Color saturation
HP Indigo ElectroInk
All HP Indigo digital presses use ElectroInk, HP Indigo’s unique liquid ink.
ElectroInk contains electrically charged ink particles, dispersed in liquid.
Similar to DEP, ElectroInk enables digital printing based on the application of
strictly controlled electrical fields to move charged color particles. This
control enables accurate placement of the printing material. However,
unlike DEP, ElectroInk enables the use of very small particle size, down to
1-2 microns. These small particles dispersed in the liquid carrier allow for
higher resolution, uniform gloss, sharp image edges, and very thin image
layers.
with the drying requirements of fast heavy coverage printing. This results
in a print quality versatility only offered by LEP.
The thin image layer closely follows the surface topography of the paper.
This gives a highly uniform finish, complementing the paper and resulting in
a similar texture both on the image and on the non-image areas.
•• HP IndiChrome spot colors - mixed from a set of base inks, matching
spot colors including most of the PANTONE color range.
For the two other digital technologies, DEP and inkjet, the situation is less
favourable. In DEP, the particle size cannot be made too small, as particles
then become airborne, and uncontrollable. Therefore, higher printing
speeds require larger particle sizes leading to adverse impact on print
quality attributes such as color performance, gloss uniformity, and
sharpness. Inkjet technology, while still laying a thin layer of ink on the
media, suffers from the inherent inaccuracy of the ink jetting. Since the ink
must strongly interact and absorb into the media, some of the pigment or
dye sinks below the surface rendering it less effective and reducing
saturation (or alternatively increasing cost). Part of the absorption follows
paper fibers leading ink away from the original droplet thus breaking the
edge of the drop in a less controlled and random manner (”wicking”). These
effects result in reduced control of the final product and with it loss of
sharpness and resolution.
HP Indigo ElectroInk is supplied in a concentrated form. Inside the press it
is fed into ink supply tanks, diluted with oil and combined with a charging
control fluid, to form a fluid mixture of carrier liquid and colorant particles
ready for printing.
The capability to increase the number of ink layers to more than four,
which is natural in offset and in ElectroInk, is very difficult, if at all
possible, when using the thick layers of DEP or, for inkjet, as it conflicts
4
HP Indigo ElectroInk is available in an increasing range of colors, including:
•• Standard CMYK (cyan, magenta, yellow, black) process colors.
•• HP IndiChrome wide-gamut 6- and 7- color sets. These incorporate
orange and violet inks for the 6 colors and also green for 7 colors to
extend the color reproduction capabilities far beyond the range possible
with CMYK inks only.
•• White ink produces an opaque appearance and solid backing for labels
and flexible packaging and also supports specialty colored and
transparent substrates for commercial applications. Premium white
with higher opacity levels (from 59% to 81%) enables the matching of
any other white printing technology including screen printing.
•• Light cyan, light magenta, light black and light light black deliver
smoother tone transitions and saturated colors for photo prints,
competing with silver halide quality.
•• Transparent Ink applied in multiple layers permits raising the surface of
the ink above the image level, creating eye-catching raised print
designs and digital watermarks, or, applied directly to the Blanket, it
builds up a ‘mold’ which imparts a textured effect to the substrate
similar to an embossed look and feel.
•• Invisible Red prints text or barcodes visible only under UV light.
•• Fluorescent Pink glows under UV light to create eye-catching effects.
•• Fade Resistant Yellow and Magenta Inks are specially designed for
outdoor applications and have an excellent lightfastness-rating of
between 6 -7, out of a maximum of 8, on the BWS (Blue Wool Scale).
To summarize, ElectroInk enables high quality, wide and accurate color
gamut, sharp images, and color with gloss closely matching the media,
similar to and at times surpassing conventional offset printing, and
exceeding the quality achieved by competing digital printing technologies.
Thermal Offset
Color switching
The HP Thermal Offset process uses a heated blanket causing the specially
shaped pigment-carrying particles within the ElectroInk to melt and blend
into a smooth film. When it contacts the cooler substrate, the ElectroInk
strongly adheres to the substrate, immediately solidifies and transfers with
almost no change in dimension or shape. Since the image is completely
defined on the blanket, issues such as ink media interaction or ink-ink
interaction which are common in nearly all other printing methods are
virtually nonexistent in LEP. The result is an image with a true offset look
and feel, accurately replicating the gloss and texture of the substrate on
which it was placed.
As the HP Indigo LEP technology employs only contact transfers and since
the HP Indigo ElectroInk does not tend to become airborne as DEP toner,
the result is a process which can inherently be much faster than the DEP
process and provide much higher quality than inkjet at the same speed.
Currently the process speed, or image generation speed, is up to 2.35 m/
sec or 462 ft/min with the release of the new HP Indigo 10000 Digital Press
platform. This inherent speed is combined with the fact that HP Indigo’s
Digital Offset Color printing technology enables the printing of all color
separations by a single engine. After one color separation has been created
and printed, the next one (usually a different color) is created and printed on
the same engine. This is possible since the blanket completely transfers the
previous image, and none of the image stays on the blanket.
Since the fusing and drying is done on the blanket and the transfer is by
contact, in contrast with DEP printing, there is little need for environmental
control of the media prior to printing or to expose the media to extreme
heat which limits the media types and may warp the media. LEP does not
require the image to be dried on the media, as in offset printing and in inkjet
printing. The print is effectively dry as soon as it leaves the press,
eliminating the risk of ink set-off marking other copies. Thus, print finishing
may be performed immediately. This is a major benefit over conventional
lithography which requires either assisted drying systems, or a natural
drying time of several hours, before any print finishing processes can be
applied.
In summary, HP Indigo’s Digital Offset Color technology enables the look
and feel of conventional offset printing, the ability to print on a virtually
limitless substrate range, and the capability of immediate drying, which
enables duplex printing or finishing with no waiting period.
Single engine printing has several obvious advantages, including
compactness, lower cost of hardware, and better mechanical accuracy, but
beyond that it offers the flexibility to balance quality with color content and
color accuracy on the same press and even within the same run.
The inherent flexibility of on-the-fly Color Switching technology means that
using the same single engine, the press is free to vary the number of color
separations to achieve the desired outcome. Whether it is adding extra
colors to widen the color gamut, introducing multiple passes of the same
color to increase opacity or create special effects, or even reducing the
number of separations to increase productivity in Enhanced Productivity
Mode, each of these outcomes can be achieved seamlessly on the same
single engine, offering unparalleled flexibility.
To summarize, the color switching technology offers an optimal balance
between speed and enhanced quality and at the same time minimizes
press footprint and cost of hardware.
Figure 4
Various applications printed on an HP Indigo digital press
5
Step-by-step description of the HP Indigo Digital Offset
Color printing process
1. Charging station
2. Laser exposure
3. Binary Ink Developer units (BIDs)
2
4. First transfer (PIP to blanket)
1
5. Blanket heating
7
3
6. Second transfer (blanket to substrate)
7. Photoconductor cleaning station
5
4
6
Figure 5
HP Indigo digital press printing cycle
The printing cycle
The HP Indigo printing engine performs the following operations
sequentially:
discharge effect (i.e. the ionization of the air) through the application of
high-voltage.
1. Electrostatic charging of the electrophotographic Photo Imaging Plate
(PIP) which is mounted on the imaging cylinder.
The negatively charged particles are directed by electric voltage toward the
PIP while the positive charged particles are attracted to the charging device
and neutralized.
2. Exposure of the PIP by a scanned array of laser diodes. These lasers are
controlled by the raster image processor which converts instructions
from a digital file into ”power” instructions for the lasers.
3. Image development performed by the Binary Ink Developer (BID) units.
4. Transfer of the inked image to the blanket cylinder.
5. Heating of the inked image carried by the blanket forming the final image
in form of a thin tacky film.
6. Complete transfer of the final image film to the substrate held by the
impression cylinder.
7. Removal of any residual ink and electrical charge from the PIP and
cooling of the PIP after engagement with the hot blanket.
These operations repeat themselves for every color separation in the
image. They are described below in more detail:
1. PIP charging
The first step in LEP is the deposition of a uniform static electric charge on
the photoconductor. This is achieved by a charge roller which produces
charged particles (atoms, molecules and free electrons) by a glow
6
In order to maintain the process stability, the voltages applied to control the
transfer of the charges to the PIP are routinely automatically calibrated to
accommodate for changes in the photoconductor’s discharging level.
2. PIP exposure
As the PIP cylinder continues to rotate, it passes the imaging unit where as
many as 32 laser beams in parallel expose the image area, dissipating
(neutralizing) the charge in those areas. When the exposed PIP rotates
toward the next station it is carrying a latent image in the form of an
invisible electrostatic charge pattern conforming to the image to be printed.
3. Image development
Inking is performed by the Binary Ink Developer (BID) units, one for each ink.
The BID units prepare a thin film of highly electrically charged ElectroInk on
their roller surface.
During printing the appropriate BID roller engages with the PIP cylinder. The
electrical fields between the PIP and the BID result in attracting the ink
paste to the image area and repelling it from the non-image areas, shearing
the ink film accurately and instantaneously.
The result is the replication of the electrical latent image with a clean
and sharp inked image.
4. First transfer
The PIP then rotates into contact with the electrically charged blanket on
the transfer cylinder, and the ink layer is electrically transferred to the
blanket.
5. Film formation (blanket heating)
The inked image is heated by the rotating thermal blanket and from an
external heat source. This causes the ink particles in the ElectroInk to
partially melt and blend together. At the same time most of the carrier oil is
evaporated, to be collected and reused as part of fresh ink in the tanks. The
result is a ready finished image in form of a hot, nearly dry, tacky plastic
film.
6. Second transfer
As the ink comes into contact with the substrate, the temperature which is
significantly below the melting temperature of the particles, the ink film
solidifies, sticks to it, and completely peels off from the blanket, ensuring
100% transfer from blanket to substrate. The blanket is therefore clean and
ready to accept the next impression with its new ink layer.
The second transfer method differs according to printing mode. In MultiShot the substrate stays on the impression cylinder as it receives each
separation from the blanket one after the other. As the final separation is
printed, the substrate is either moved for duplex printing or delivered to the
output tray. In One Shot mode, the PIP cylinder transfers a succession of
separations to the blanket before they are transferred to the substrate in a
single pass. One Shot is standard for Labels and Packaging presses,
whereas most Commercial presses have the option for using either mode,
depending on the substrate.
7. Cleaning station
Returning to the PIP, after transferring the image to the blanket, it rotates
past a cleaning station which removes any residual ink and cools the PIP
from heat transferred during contact with the hot blanket. At this point this
part of the PIP surface has made a complete rotation and can be recharged
ready for the next image.
As mentioned before, HP Indigo presses print multiple colors from the same
offset blanket. The cycle repeats itself for each color separation and the
only difference between the cycles is in the ink application and the image
data corresponding to the printed color separation.
7
Digital Offset Color advantages
After describing the technology and the process, let’s touch on the
advantages which stem from them.
Figure 6
Comparison between dense lines
Quality characteristics
1. Edge sharpness and definition
Figures 8 and 9 display microscope images of dots printed with HP Indigo
ElectroInk, conventional offset technology, and DEP technology. All prints
have been calibrated to produce the same final color value. Viewed at high
magnification, ElectroInk forms much sharper features than any other
printing method. For small dots, the conventional offset creates
“smudges” due to the ink splitting, while DEP looks like a pile of powder
with regions flattened by the fusing and uncontrolled dust between the
dots.
The sharpness of ElectroInk is particularly noticeable at the edges of
halftone dots, or fine type characters (see figure 6 and figure 7).
Noticeable is the contamination-free background of ElectroInk images.
This is first due to the small size of the ink particles and also to the way
ElectroInk particles are transformed when they are on the press. As
opposed to both DEP and inkjet, ElectroInk is not transported by flight
though air, but rather a thin ink paste is placed on the PIP and “cut” to the
correct size. This produces very accurate edges and almost no
background contamination. In the printing process ElectroInk paste is
transferred by contact to the heated blanket, because the ink particles
melt and blend, the strong surface tension of the liquefied heated ink,
while becoming a film, facilitates the formation of a sharp, clean edge.
When transferred from the blanket to the final printing substrate (paper
or plastic), the heated plastic film cools down to form a thin colored
plastic layer on the substrate surface. When printing on paper, the cooled
ElectroInk does not soak (or wick) into the paper fibers. Thus, printed
dots, linework, and text stay sharp and well defined on the surface of the
paper.
Like ElectroInk, DEP toners don’t penetrate the paper either, but they do
suffer from large particle size and stray toner particles scattered outside
the image edges, leading to poor edge definition no matter what
substrate is used.
8
HP Indigo ElectroInk
DEP
Figure 7
Comparison of text printed with HP Indigo digital offset
(left) with text printed by DEP (right)
Comparison of text printed with HP Indigo digital offset (left)
with text printed by industrial inkjet press (right)
Figure 8
Compare ElectroInk (left),
offset (middle) and DEP (right).
Figure 9
Comparison between color dots printed with
HP Indigo ElectroInk (left) and DEP (right)
LEP
2. Optical density, dot gain color and image consistency
Optical density—the amount of light which the ink absorbs, or its
darkness is defined by the thickness of the ink on the media and the
concentration of light absorbing material (pigment in our case) it contains.
Dot gain is the difference in dimension between the dots and lines as they
were designed in the original digital file and their final dimension on the
final print. In combination, these two elements are the basis for print
stability. For HP Indigo digital presses, both optical density and dot gain
are periodically monitored and press parameters are automatically
adjusted to minimiz`e fluctuation. The result is color and linework that are
consistent and predictable over time and between presses. HP Indigo
presses automatically adjust the optical density (i.e. appearance in terms
of lightness or darkness) and dot size so that they always appear the
same, copy after copy, regardless of small changes in press parameters
and even in uncontrolled changes in media batch. On top of this HP Indigo
presses have built-in dot gain compensation which corrects the exposed
dot size and thickness so that it prints to the color profile with the media
included in the equation.
With a conventional offset lithographic press, there are wider fluctuations
during a run caused by factors that include: fluctuating ink and water
temperature; water/ink balance and their tendency to emulsify; plate and
blanket wear; and atmospheric humidity altering the absorbency of the
DEP
paper. Neither automatic nor manual adjustment can completely iron
these out, because there is always a time lag between the problem
appearing and the adjustment taking effect, during which many copies
are printed. With HP Indigo presses, there are fewer operating variables,
and the optical density of the printed image can be electronically set by
the operator within a wide operational range.
The capability of printing to a specific color profile is a necessary
requirement in order to achieve print consistency. A special capability of
the HP Indigo presses is the ability to print linework and fonts with nearly
zero dot gain. This is achieved by combining a per-pixel variable laser
power capability together with the previously discussed inherent
sharpness and accuracy of the ElectroInk process. The combined result of
accurate color and linework is a print which is consistent from print to
print and press to press.
Just as importantly, the parameters which created the print, such as
optical density, color profiles and screen, are kept for repeat runs in
future, meaning that a repeat job will be identical to the original
regardless of the press or media batch. This is difficult to achieve with
lithography, which depends to a great extent on operator skill.
9
Figure 10
Gloss comparison for various printing systems
3. Imaging flexibility
HP Indigo presses have been created to offer, by design, full flexibility. The
setting of crucial elements such as optical density, color curve, screen and
even dot gain may be done before the RIP (Raster Image Processing), but
may also be done just before printing by a touch of the keyboard or touch
screen and the image will be reprinted with the new required conditions,
without re-RIP or time lag. Since the printed ink layers are thin and do not
interact, there is little need to change color profile when changing screen.
This allows quick proofing cycles when proofing is required, in turn
shortening time and minimizing waste of media.
100
80
Xerox iGen4
70
NextPress 2100
60
50
40
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Image gloss%
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Obviously most of these capabilities are lacking with lithography, but even
DEP is limited by the fact that the thick toner layers strongly interact (or
trap) each other and thus any change in screen, optical density, dot gain
or color curve will require a new RIP to receive a reasonable color output.
Ink
30
20
10
0
0
10
20
30
40
50
60
Paper gloss %
70
Paper roughness
Figure 11
HP Indigo ElectroInk complements the topography of the media
80
90
100
4. Image gloss
Many people believe that the key to the quality of offset litho printing is its
glossy appearance. This isn’t quite true - what is important is the uniformity
of the gloss and matching with the media. For instance, many magazines
and company brochures have varnished front covers to give a high quality
feel, but either gloss or matte varnishes may be used. In addition, some of
the most expensive looking substrates have intentional textures which
should not be glossed over.
The uniformity of the finish is the key issue rather than just its surface gloss.
HP Indigo ElectroInk gives a highly uniform finish complementing that of the
underlying substrate - whether the substrate is a high gloss coated paper or
a rough matte paper.
ElectroInk images match the gloss of the underlying printing substrate,
from rough to dull to high gloss, just like conventional offset prints. Paper
stocks have a typical surface roughness ranging from about 1 to 10 microns
in height. The ElectroInk layer is only about 1 micron thick, and therefore it
follows the ”hills and valleys” of the substrate surface texture, rather than
filling them in. The result is that there are no large variations in gloss
between the inked image areas and the bare paper substrate
(see figure 11).
Paper
Paper covered
with HP Indigo ElectroInk
10
Even the finest DEP color toner is limited to a particle size no lower than 5 to
9 microns, otherwise it is too small to be controllable and forms a powder
cloud or dust. Since powder toner particles are so large, they create thick
images - which cannot replicate the surface roughness of the paper. Powder
toner images therefore have their own unique gloss which contrasts with
the gloss of the paper. This gloss non-uniformity is perceived as poor quality
printing. Plotting the gloss characteristics of the various digital and
lithographic processes on a graph demonstrates that ElectroInk has
practically ideal reflective characteristics, nearly matching the paper gloss
for all but the very smoothest coated surfaces. DEP toners plot as straight
horizontal lines on the graph, indicating they have their own gloss no matter
what the substrate.
5. Color gamut
HP Indigo ElectroInk primary colors are similar to the ones defined in
ISO 12647-2. This similarity allows the transfer of jobs between offset
presses and Indigo digital presses with either minimal or no color
transformations. The conformity with offset, in turn, enables the
combination of digital and offset pages and the possibility to replace long
offset runs (and the accompanying storage requirements) with a few short
digital runs. The nature of the ElectroInk process earned HP Indigo digital
presses GRACoL proofing and Fogra production certification, confirming
their ability to create accurate color prints and work with well defined color
standards for color printing.
ElectroInk is easily calibrated for out-of-standard thickness and dot gain
supporting the use of different color profiles and removing the need to
change ink per application. Therefore ElectroInk can use one CMYK color set
for all applications, unlike lithography which requires many different CMYK
ink formulations.
Beyond the four process colors (CMYK), HP Indigo digital presses may utilize
up to three additional colors in the HP IndiChrome set (CMYK, orange, violet)
or IndiChrome Plus set (CMYK, orange, violet and green) increasing available
color gamut.
For very discerning applications which require accurate spot color, the
unique HP Indigo Ink Mixing System (IMS) provides great flexibility to
customers wishing to match specific PANTONE spot colors. The system
allows users to mix, on site, a wide range of special colors from a set of 11
base inks, with fully automated software guiding the user through the
measurement, analysis, and mixing steps. (See color gamut schematic on
page 3).
6. Instantaneous image drying
Because ElectroInk solidifies as soon as it transfers to the substrate, the
finished print emerges dry from the HP Indigo printing press. Further image
hardness is acquired in the first few hours post printing.
HP Indigo printed material can withstand considerable handling activities
immediately, unlike conventional offset lithography, not equipped with
assisted drying, which requires a drying period of several hours before
further processes, such as cutting or folding, can be performed. Print drying
is considered by GATF (The Graphic Arts Technical Foundation) as the
Figure 12
HP Indigo print permanence
maximum longevity
compared with silver halide
number one problem of conventional lithographic printing, responsible for
many image artefacts and defects, as well as print operation issues.
Both DEP and inkjet also produce ready to handle products, nevertheless
the extreme heat needed for DEP to fuse on the paper or the requirement
to absorb the ink of inkjet into the paper and then to dry it on the paper
strongly limit the media selection for these technologies.
7. Lightfastness
The encapsulation of the pigment sub-particles within the ElectroInk
plastic resin helps preserve the chemical properties of the pigments
against oxidation and humidity effects, especially under strong ultraviolet daylight conditions. This means that color durability of printed
images, either in the form of fading or deepening, is superior compared to
conventional offset printing.
Photo consumers are also seeking alternatives to traditional photo printing
with silver halide chemicals. In new testing* conducted by Wilhelm Imaging
Research (WIR), HP Indigo prints exceeded or matched the display image
permanence ratings of the best-rated silver halide product. The WIR testing
also validated the album permanence of HP Indigo photobook pages, which
received the highest possible dark permanence rating of > 200 years. In
comparison, the WIR gave all the silver halide photos a much lower album/
dark storage rating of > 100 years.
* Results published on March 7, 2011
200
HP Indigo Album Permanence
Silver Halide
HP Indigo Display
Permanence
Fujicolor Crystal
Archive
Kodak
Edge
Generations
0
54
50
19
200 years
11
Substrate compatibility
1. Variety
HP Indigo’s Digital Offset Color process is compatible with a wider variety
of substrate types, surfaces and thickness than any other digital printing
process. These include paper, card stock, plastic, film, paperboard, and
metals. Only one formulation of HP Indigo ElectroInk is needed to print on
any stock that the press can handle. This means that HP Indigo press
users can rapidly switch between substrates without having to worry
about changing inks. With conventional offset printing, different inks for
papers and non-absorbing plastic films are required; the latter usually
need UV polymerizable inks. When printing on paper stocks with different
absorbencies, it may be necessary to adjust the viscosity of the ink by
means of thinning or thickening agents, or even use specially formulated
inks. Particularly absorbent papers can also increase offset ink
consumption by up to about 50%, which contrasts with ElectroInk where
consumption has almost no dependence on the substrate properties.
DEP technology is heavily dependent upon the electrostatic properties of
the paper substrate, and small changes in the environmental relative
humidity may result in noticeable variations in print quality. Moreover, the
high fusing temperature needed for DEP puts serious limitations on the
choice of coated paper stock or plastic films that can be printed.
Inkjet suffers from even a stronger limitation due to the requirement to
absorb the ink into the media. Plastics are out of the question unless
dealing with UV inks or coating the media with a coating that will, in effect,
absorb the ink.
This flexibility in media is what led HP Indigo industrial presses to be the
leading source for digitally printed labels and also serve as a replacement
technology for flexography. For commercial printing, if there is a need to
print on very rough surfaces or off-the-shelf offset papers, this can be
achieved by the application of HP Indigo ElectroInk Primer. The primer
uses a special additive that chemically binds to the paper on one side and
the colored ink on the other, thus strengthening the link between the two.
The primer is contained within a transparent ink that can be installed in a
normal ink station on the press. As it is applied as an ink, the DFE simply
creates an additional separation that selectively applies the ElectroInk
Primer to only those areas of the substrate where the image appears. The
result is an almost limitless range of substrates, including standard offset
stocks and specialty media.
2. Lamination and over-varnishing
Stock printed with ElectroInk is compatible with standard coating
processes such as lamination or varnishing. Lamination of thin plastic
films over the printed stock can be done in the conventional way, using a
variety of solvent-born, water-based, UV-based or solvent-free adhesives.
A varnish coating, either UV or water-based, can also be used and
ElectroInk plastic resin withstands a large variety of chemical solvents. No
significant image degradation occurs when using most of the standard
coating materials.
Applications printed in Enhanced Productivity Mode, maintaining HP Indigo’s high print quality at higher productivity and lower cost.
12
CMYK
EPM
Figure 13 Compare CMYK color dot prints to Enhanced
Productivity Mode printing in three colors.
Productivity and versatility
Enhanced Productivity Mode (EPM) takes advantage of the inherent
flexibility of the HP Indigo Digital Offset process to deliver improved
productivity for only a modest, likely unnoticeable, reduction in the color
gamut (primarily in the very dark areas of the spectrum). As all color
separations on HP Indigo digital presses are produced on a single engine,
color jobs can be printed 33% faster in CMY while eliminating the black (K)
separation. Complex algorithms in the Print Server’s color management
process translate the black (K) separation information in to the remaining
CMY separations, generating the black effect by overlapping the three
remaining colors. The result is indistinguishable to the eye from standard
4 separation CMYK (small differences may be noticeable in thin black lines
and very small black text), and is suitable for any but the highest quality
print jobs.
In addition, eliminating black (K), has been shown to result in some
improvements in print quality (increased smoothness, reduced
graininess). These improvements can be enhanced even further in
combination with light colors light inks (light black, light light black, light
cyan, light magenta) which deliver smoother tone transitions and
saturated colors. This is particularly advantageous in Photo applications,
to achieve silver halide quality.
Rules-based pre-flight solutions can automatically filter print-ready PDF
files to select those best-suited to EPM, communicating with the HP
Indigo DFE to insure that the files are printed in the correct printing mode,
making its deployment simple and effortless. EPM also determines
whether any sheets in a job contain only black, in which case they are
processed separately as black-only (K) with no additional colors. This
allows EPM to be efficient even when pages in the job are black-only.
13
Color management
Future capabilities
Indigo has developed a suite of integrated tools designed to simplify and
automate the color management process. These tools ensure that the
exceptional Indigo color accuracy remains consistent and uniform across
presses and sites and over time, without laborious manual calibration.
HP Indigo continues to develop and enhance the technical implementation
of its core Digital Offset printing processes to maximize its potential.
Developments such as the new High Definition Laser Array (HDLA) writing
head, due to be launched in 2017, are taking Indigo’s already exceptional
print quality to a new level. The new Writing Head increases the number
of lasers in the array from 28 to 40, angles their deployment to halve the
space between the image of each laser on the photoconductor, and
increases Polygon speed by 140% to keep up with the process
requirements. The result is a doubling of the precision with which each
dot can be placed on the substrate for an improved addressability of
1600 dpi. This doubling of addressability enables both smoother screens
with 180 lpi, a standard in Indigo printing, and new screens at 220 lpi, with
smaller, nearly invisible rosettes. The main benefits are an increase in
image smoothness, the elimination of graininess, and sharper, more
accurate features. The impact can best be visualized as the equivalent of
a four-fold improvement in digital camera resolution (e.g. 1.5MP – 6MP).
Tests of Indigo’s print quality with 220lpi at 1600dpi have indicated that it
now surpasses offset lithography.
The Indigo Color Management system starts with the media; an Inline
Spectrophotometer scans a color chart to describe the individual color
space of a specific substrate (the “Media Fingerprint”). The DFE (Digital
Front End) then automatically generates a dedicated ICC profile, ensuring
accurate color matching of that specific substrate against a given FOGRA,
GRACOL etc. tool based on standard such as
ISO 12647-2.
Accurate matching to these industry standards also enables seamless
emulation of output from other print technologies (such as offset).
A three-dimensional LUT is then used to regularly calibrate the press,
automatically compensating for any variations in press conditions to
return it to the baseline established by the Media Fingerprint, ensuring
that color output always remains consistent. On web-fed presses this
process happens at a pre-determined interval, whereas onsheetfed
presses the operator is free to determine the frequency of calibrations.
Media Fingerprints can be saved and shared between presses and sites,
and over time, to ensure a consistent output regardless of where or when
the file is printed. HP PrintOS allows the easy and rapid sharing of
substrate files via the cloud.
HDLA technology enables finer printing with 220 lpi screen (right).
14
Summary
The HP Indigo Digital Offset Color printing process is the only variable
imaging printing technology that can equal or exceed the quality, color
range, and substrate compatibility of conventional offset lithographic
printing, as well as flexographic printing, and is also the best choice for
replacing silver halide technology photo printing.
While competing with offset printing for print quality, the digital
technology offers important benefits, both economic and environmental.
Since each product may be targeted at a final recipient, waste is reduced
and warehousing is minimized as pages may be printed just in time and
on demand. Moreover, the HP Indigo digital process offers cost
effectiveness and is environmentally friendly as it both reduces waste and
eliminates the use of hazardous chemical materials.
Apart from the HP Indigo Digital Offset Color process, the other main
variable imaging digital color press technologies are the DEP process and
inkjet presses. The limitations of dry toner printing have been detailed
above, but to recap briefly: fine detail and acceptable colors can only be
achieved with very small pigmented particles. Dry toner particles have to
be above a critical size of at least 5 to 9 microns otherwise they form a
dust cloud and cannot be controlled in the press. Some DEP processes can
produce high gloss toner images, but they cannot match the gloss of the
substrate surface, thereby producing high “gloss contrast” which is
perceived as poor quality. HP Indigo’s liquid ElectroInk uses oil to bind and
distribute its pigment-carrying particles, which are about 1 micron in size,
and consequently capable of creating much finer detail and thinner
printed films. DEP presses, with their inherently limited process speed,
require multiple printing units where the only flexibility is the decision of
whether a unit engages the media or not. Therefore an important
consideration when making comparisons between these and the single
unit HP Indigo presses is simplicity. HP Indigo’s color switching technology
enables single station printing presses, which results in flexibility, more
compact presses, less parts to maintain, and less potential for things to
go wrong.
For inkjet technology, the comparison parameters are different.
inkjet technology requires both that the ink absorbs into the media and
that vast amounts of liquid are evaporated from the media after the ink is
placed. This severely narrows the substrate range and the image
coverage of high speed inkjet devices. When combined with the inherent
inaccuracy of the jetting and wicking, the result is a technology which,
though very adequate for certain applications, is less versatile and less
flexible and cannot compete for overall quality or for overall media range
with the HP Indigo Digital Offset Color process.
To summarize, the HP Indigo digital printing process offers a unique
combination of quality, versatility, and productivity unmatched by any
other existing digital technology. The key considerations for organizations
investing in digital color printing are quality, speed, total costs (fixed and
variable), versatility, and product range, and though for some niche
markets other solutions may be equally advantageous, HP technology
places it as a leading contender in all these respects.
15
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